The reason why many other VST instruments works with PC messages, is because they still operates as if they were MIDI instruments (like an external hardware synthesizer with MIDI). They need to do this in order to work with the many hosts available, since there are hosts that do not have a built-in preset system.
This is why you often see two patch/preset/program systems in VST plug-ins, one that seems to be part of it and the other being the native Cubase VST Preset system.
Anyways, some of these VST instruments respond to bank messages to handle more than 128 programs (the limit of the Program Change event message), others use a working “bank” that you fill with 128 programs for the instance, and some have a limit of 128 programs, etc. There are many ways that this can and is being done.
So why not Padshop and Retrologue then? Well, I do not know the real reason of course, but my guess is that Steinberg probably have bigger plans for its VST Preset system, which is the one available at the top of a VST instrument window (load/save preset, etc.) and used in MediaBay. So with the ability to use more tracks rather than complicate a single track with PC events, that’s the road chosen for (some of) their VST instruments.
Steinberg has not removed support for the MIDI style PC functionality entirely, e.g. HALion 4 still supports the 128 programs per instance.
Who knows, maybe there will be a new VST3 Preset/NE Program Change…